Recently, consumer electronics are becoming increasingly popular. Capacitive touch panels have become indispensable input interface for consumer electronics, so as to allow users to operate the electronic devices easily. The electronic devices are capable of detecting touch points depending upon capacitance variation generated when the capacitive touch panels are touched by the users.
Referring to FIGS. 1(A) and 1(B), there are shown circuit structure diagrams of a conventional capacitive sensing circuit, respectively. As illustrated in the figures, the capacitive sensing circuit 100 is applied in an electronic device having a capacitive touch panel. The capacitive sensing circuit 100 is used for sensing capacitance variation generated when the capacitive touch panel is touched by means of charge transfer.
The capacitive sensing circuit 100 comprises a capacitor to be measured (CX) 11, an integral capacitor (CINT) 12 and a switch 13, in which capacitance of the integral capacitor 12 is much higher than capacitance of the capacitor to be measured 11. The capacitor to be measured 11 is composed of at least one touch capacitor (CTOUCH) and at least one parasitic capacitor (CPAD, CITO), where CX=CTOUCH+CPAD+CITO. When the capacitive touch panel is touched, capacitance of the touch capacitor (CTOUCH) is varied, such as, 0 pF→1 pF, along with touch operations. Furthermore, the one end of the switch 13 is connected to the capacitor to be measured 11, while the other end of the switch 13 is selected to switch to the power supply (VDD) or the integral capacitor 12.
The capacitive sensing circuit 100 is started to perform charge transfer process as follows. Firstly, as illustrated in FIG. 1(A), the switch 13 is controlled to switch to the power supply VDD, allowing for a charging current |C generated from the power supply VDD to charge the capacitor to be measured 11. Subsequently, as illustrated in FIG. 1(B), the switch 13 is controlled to switch to the integral capacitor 12 after the capacitor to be measured 11 is fully charged, allowing for discharging the capacitor to be measured 11. A discharging current |D from the capacitor to be measured 11 is used to charge the integral capacitor 12. The charge energy of the capacitor to be measured 11 is then transferred to the integral capacitor 12, such that a voltage signal VINT is generated on the integral capacitor 12.
The switch 13 may be controlled by the capacitive sensing circuit 100 to switch between the power supply VDD and the integral capacitor 12 several times repeatedly, so as to enlarge capacitance variation of the capacitor to be measured 11, due to extremely small capacitance variation generated on touch of the capacitor to be measured 11. Then, charge energy charged on the capacitor to be measured 11 may be transferred to the integral capacitor 12 several times, such that the voltage signal VINT may be accumulated to be enlarged.
Subsequently, referring to FIG. 2, there is shown a curve diagram of voltage signal generated on the integral capacitor by the conventional capacitive sensing circuit. In this case, CPAD+CITO=25 pF, CTOUCH=0 pF→1 pF, CINT=100 pF may be taken as the operating standard of the capacitive sensing circuit 100. Before the capacitive touch panel is touched, capacitance of the capacitor to be measured 11 is CX=25 pF. A pre-touch voltage signal curve 120 is obtained through repeated charge transfer processes performed between the capacitor to be measured 11 and the integral capacitor 12. After the capacitive touch panel is touched, the capacitor to be measured 11 is varied as CX=25 pF→26 pF. The other post-touch voltage signal curve 121 is obtained through another repeated charge transfer processes additionally performed between the capacitor to be measured 11 and the integral capacitor 12.
The post-touch capacitor to be measured (CX=26 pF) 11 may be fully charged with charge energy, which is more than that fully charged on the pre-touch capacitor to be measured (CX=25 pF) 11. Therefore, more charge energy may be transferred from the post-touch capacitor to be measured (CX=26 pF) 11 to the integral capacitor 12, such that the potential of post-touch voltage signal curve 121 may be higher than that of pre-touch voltage signal curve 120. Moreover, when the difference in potential between the voltage signal curves 120 and 121 is larger than a predetermined difference, capacitance variation generated when the capacitive touch panel is touched may be sensed by the electronic device.
In the manner of charge transfer, charge transfer is carried out by means of voltage difference between the capacitor to be measured 11 and the integral capacitor 12 primarily. At the beginning of charge transfer process, the voltage difference between the capacitor to be measured 11 and the integral capacitor 12 is the largest, such that a higher discharging current |D may be provided by the capacitor to be measured 11 for charging the integral capacitor 12, thus transferring charge energy of the capacitor to be measured 11 to the integral capacitor 12 entirely, resulting in a larger extent in raising the potential of the voltage signal VINT. In the continuous charge transfer process, the discharging current |D becomes smaller as the voltage difference between the capacitor to be measured 11 and the integral capacitor 12 is smaller and smaller, such that transferring charge energy from the capacitor to be measured 11 to the integral capacitor 12 is harder and harder, resulting in smaller and smaller extent in raising the potential of the voltage signal VINT. The capacitive sensing circuit 100 is then necessary to perform charge transfer process more times, due to the smaller and smaller extent in raising the potential of the voltage signal VINT, so as to enlarge the difference in potential between the voltage signal curves 120 and 121 over the predetermined difference. Thereby, time for sensing capacitance variation is prolonged correspondingly, thus being detrimental to real-time operation on touch.